Production of Liquid Hydrocarbons from Methane

ABSTRACT

In a process for converting methane to liquid hydrocarbons, a feed containing methane is contacted with 0 de-hydrocyclization catalyst under conditions effective to convert said methane to aromatic hydrocarbons, including benzene and/or naphthalene, and produce a first effluent stream comprising hydrogen and 0t least 5 wt % m&gt;35 aromatic hydrocarbons than said feed. At least part the aromatic hydrocarbons from the first effluent stream is then reacted with hydrogen to produce a second effluent stream having a reduced benzene and/or naphthalene content compared with said first effluent stream.

FIELD

This application describes a process for producing liquid hydrocarbonsfrom methane and, in particular, from natural gas.

BACKGROUND

Transportable liquid hydrocarbons, such as cyclohexane and decalin, areimportant commodities for fuel and chemical use. Currently, liquidhydrocarbons are mostly frequently produced from crude oil-basedfeedstocks by a variety of processes. However, as the world supplies ofcrude oil feedstocks decrease, there is a growing need to findalternative sources of liquid hydrocarbons.

One possible alternative source of liquid hydrocarbons is methane, whichis the major constituent of natural gas and biogas. World reserves ofnatural gas are constantly being upgraded and more natural gas iscurrently being discovered than oil. Because of the problems associatedwith transportation of large volumes of natural gas, most of the naturalgas produced along with oil, particularly at remote places, is flaredand wasted. Hence the conversion of alkanes contained in natural gasdirectly to higher hydrocarbons, is a particularly attractive method ofupgrading natural gas, providing the attendant technical difficultiescan be overcome.

A large majority of the processes for converting methane to liquidhydrocarbons involve first conversion of the methane to synthesis gas, ablend of H₂ and CO. Production of synthesis gas is capital and energyintensive; therefore routes that do not require synthesis gas generationare preferred.

A number of alternative processes have been proposed for convertingmethane directly to higher hydrocarbons. One such process involvescatalytic oxidative coupling of methane to olefins followed by thecatalytic conversion of the olefins to liquid hydrocarbons, includingaromatic hydrocarbons. For example, U.S. Pat. No. 5,336,825 discloses atwo-step process for the oxidative conversion of methane to gasolinerange hydrocarbons comprising aromatic hydrocarbons. In the first step,methane is converted to ethylene and minor amounts of C₃ and C₄ olefinsin the presence of free oxygen using a rare earth metal promotedalkaline earth metal oxide catalyst at a temperature between 500° C. and1000° C. The ethylene and higher olefins formed in the first step arethen converted to gasoline range liquid hydrocarbons over an acidicsolid catalyst containing a high silica pentasil zeolite.

Dehydroaromatization of methane via high-temperature reductive couplinghas also been proposed as a route for upgrading methane into higherhydrocarbons, particularly ethylene, benzene and naphthalene. Thus, forexample, U.S. Pat. No. 4,727,206 discloses a process for producingliquids rich in aromatic hydrocarbons by contacting methane at atemperature between 600° C. and 800° C. in the absence of oxygen with acatalyst composition comprising an aluminosilicate having a silica toalumina molar ratio of at least 5:1, said aluminosilicate being loadedwith (i) gallium or a compound thereof and (ii) a metal or a compoundthereof from Group VIIB of the Periodic Table.

U.S. Pat. No. 5,026,937 discloses a process for the aromatization ofmethane which comprises the steps of passing a feed stream, whichcomprises over 0.5 mole percent hydrogen and 50 mole percent methane,into a reaction zone having at least one bed of solid catalystcomprising ZSM-5 and phosphorous-containing alumina at conversionconditions which include a temperature of 550° C. to 750° C., a pressureless than 10 atmospheres absolute (1000 kPaa) and a gas hourly spacevelocity of 400 to 7,500 hr⁻¹. The product effluent is said to includemethane, hydrogen, at least 3 mole % C₂ hydrocarbons and at least 5 mole% C₆-C₈ aromatic hydrocarbons. After condensation to remove the C₄+fraction, cryogenic techniques are proposed to separate the hydrogen andlight hydrocarbons (methane, ethane, ethylene, etc.) in the producteffluent.

U.S. Pat. No. 5,936,135 discloses a low temperature, non-oxidativeprocess for the conversion of a lower alkane, such as methane or ethane,to aromatic hydrocarbons. In this process, the lower alkane is mixedwith a higher olefin or paraffin, such as propylene or butene, and themixture is contacted with a pretreated bifunctional pentasil zeolitecatalyst, such as GaZSM-5, at a temperature of 300° C. to 600° C., a gashourly space velocity of 1000 to 100000 cm³ g⁻¹ hr⁻¹ and a pressure of 1to 5 atmosphere (100 to 500 kPa). Pretreatment of the catalyst involvescontacting the catalyst with a mixture of hydrogen and steam at atemperature 400° C. to 800° C., a pressure of 1 to 5 atmosphere (100 to500 kPa) and a gas hourly space velocity of at least 500 cm³ g⁻¹ hr⁻¹for a period of at least 0.5 hour and then contacting the catalyst withair or oxygen at a temperature of 400° C. to 800° C., a gas hourly spacevelocity of at least 200 cm³ g⁻¹ hr⁻¹ and a pressure of 1 to 5atmosphere (100 to 500 kPa) for a period of at least 0.2 hour.

U.S. Pat. Nos. 6,239,057 and 6,426,442 disclose a process for producinghigher carbon number hydrocarbons, e.g., benzene, from low carbon numberhydrocarbons, such as methane, by contacting the latter with a catalystcomprising a porous support, such as ZSM-5, which has dispersed thereonrhenium and a promoter metal such as iron, cobalt, vanadium, manganese,molybdenum, tungsten or a mixture thereof. The addition of CO or CO₂ tothe feed is said to increase the yield of benzene and the stability ofthe catalyst.

U.S. Pat. No. 6,552,243 discloses a process for the non-oxidativearomatization of methane, in which a catalyst composition comprising ametal-loaded, crystalline aluminosilicate molecular sieve is initiallyactivated by treatment with a mixture of hydrogen and a C₂ to C₄ alkane,preferably butane, and then the activated catalyst is contacted with afeed stream comprising at least 40 mole percent methane at a temperatureof 600° C. to 800° C., a pressure of less than 5 atmosphere absolute(500 kpaa), and a weight hourly space velocity (WHSV) of 0.1 to 10 hr⁻¹.

Russian Patent No. 2,135,441 discloses a process for converting methaneto heavier hydrocarbons, in which the methane is mixed with at least 5wt % of a C₃+ hydrocarbon, such as benzene, and then contacted in amulti-stage reactor system with a catalyst comprising metallic platinumhaving a degree of oxidation greater than zero at a methane partialpressure of at least 0.05 MPa and a temperature of at least 440° C.Hydrogen generated in the process may be contacted with oxides of carbonto generate additional methane that, after removal of the co-producedwater, can be added to the methane feed. The products of the methaneconversion are a C₂-C₄ gaseous phase and a C₅+ liquid phase but,according the Examples, there is little (less than 5 wt %) or no netincrease in aromatic rings as compared with the feed.

Existing proposals for the conversion of methane to aromatichydrocarbons suffer from a variety of problems that have limited theircommercial potential. Oxidative coupling methods generally involvehighly exothermic and potentially hazardous methane combustionreactions, frequently require expensive oxygen generation facilities andproduce large quantities of environmentally sensitive carbon oxides. Onthe other hand, existing reductive coupling techniques frequently havelow selectivity to aromatics and may require expensive co-feeds toimprove conversion and/or aromatics selectivity. Moreover, any reductivecoupling process generates large quantities of hydrogen and so, foreconomic viability, requires a route for effective utilization of thehydrogen by-product. Since natural gas fields are frequently at remotelocations, effective hydrogen utilization can present a substantialchallenge.

An additional limitation of these technologies is that they tend toproduce predominately benzene and naphthalene as products. While benzenehas potential value as a chemical feedstock it has a limited chemicalmarket and is not viable as a fuel source due to health andenvironmental issues. Naphthalene has an even more limited chemicalsmarket and is more challenging for use as a fuel due to health andenvironmental issues plus a melting point higher than ambienttemperature.

A particular difficulty in using natural gas as a liquid hydrocarbonsource concerns the fact that many natural gas fields around the worldcontain large quantities, sometimes in excess of 50%, of carbon dioxide.Not only is carbon dioxide a target of increasing governmentalregulation because of its potential contribution to global climatechange, but also any process which requires separation and disposal oflarge quantities of carbon dioxide from natural gas is likely to beeconomically prohibitive. In fact, some natural gas fields have suchhigh carbon dioxide levels as to be currently considered economicallyunrecoverable.

There is therefore a need for an improved process for converting methaneto liquid hydrocarbons, particularly where the methane is present in anatural gas stream containing large quantities of carbon dioxide.

SUMMARY

In one aspect, this application describes a process for convertingmethane to higher hydrocarbons, the process comprising:

(a) contacting a feed containing methane and at least one of H₂, H₂O, COand CO₂ with a dehydrocyclization catalyst under conditions effective toconvert said methane to aromatic hydrocarbons, including benzene and/ornaphthalene, and produce a first effluent stream comprising aromatichydrocarbons and hydrogen, wherein said first effluent stream comprisesat least 5 wt % more aromatic hydrocarbons than said feed; and

(b) reacting at least part of said aromatic hydrocarbons from said firsteffluent stream with hydrogen to produce a second effluent stream havinga reduced benzene and/or naphthalene content compared with said firsteffluent stream.

Conveniently, said feed in (a) contains less than 5 wt % of C₃+hydrocarbons. As used herein, the term “C₃+ hydrocarbons” meanshydrocarbons having 4 or more carbon atoms.

Conveniently, said conditions in (a) are non-oxidizing conditions. By“non-oxidizing” is meant that oxidizing agents (such as, O₂, NO_(x) andmetal oxides which can release oxygen to oxidize methane to CO_(x)) arepresent at less than 5%, preferably at less then 1%, most preferably atless than 0.1%, of the amount required for stoichiometric oxidation ofthe methane.

Typically said conditions in (a) include a temperature of 400° C. to200° C., such as 500° C. to 975° C., for example 600° C. to 950° C.

Conveniently, said reacting (b) converts at least part of the benzeneand/or naphthalene in said first effluent stream to one or more ofcyclohexane, cyclohexene, dihydronaphthalene(benzylcyclohexene),tetrahydronaphthalene(tetralin), hexahydronaphthalene(dicyclohexene),octahydronaphthalene, and decahydronaphthalene(decalin).

Conveniently, said reacting (b) hydrogenates and hydrocracks at least aportion of the naphthalene in said first effluent stream to producealkylated one ring aromatic species such as ethylbenzene, xylenes,cumene, tri-methylbenzene, butylbenzene, diethylbenzene,methylethylbenzene and other typical isomers.

In one embodiment, said reacting (b) hydrocracks at least part of thebenzene and/or naphthalene in said first effluent stream to normaland/or isoparaffins.

Conveniently, at least part of the benzene and/or naphthalene in saidfirst effluent stream can be further reacted with cyclohexene,conveniently formed by said reacting (c), to respectively formcyclohexyl benzene and/or cyclohexyl naphthalene.

Conveniently, the process also comprises separating at least onearomatic hydrocarbon, typically benzene and/or naphthalene, from saidfirst effluent stream. Before or after said separating, the aromaticcompounds in said first effluent stream can be alkylated with analkylating agent. In one embodiment, the alkylating agent is ethyleneproduced in said contacting (a). In another embodiment, the alkylatingagent comprises carbon monoxide and hydrogen or a reaction productthereof, wherein the carbon monoxide is produced by said reacting (b).

In a further aspect, the this application describes a process forconverting methane to higher hydrocarbons, the process comprising:

(a) contacting a feed containing methane and CO and/or CO₂ with adehydrocyclization catalyst under non-oxidizing conditions effective toconvert said methane to aromatic hydrocarbons, including benzene and/ornaphthalene, and produce a first effluent stream comprising aromatichydrocarbons and hydrogen, wherein said first effluent stream comprisesat least 5 wt % more aromatic hydrocarbons than said feed;

(b) reacting at least part of said aromatic hydrocarbons from said firsteffluent stream with at least part of the hydrogen from said firsteffluent stream to produce a second effluent stream having a reducedbenzene and/or naphthalene content compared with said first effluentstream;

(c) reacting at least part of the hydrogen from said second effluentstream with an oxygen containing species to produce a third effluentstream comprising water and hydrocarbon; and

(d) recycling at least part of the hydrocarbon in said third effluentstream to said contacting (a).

It is to be appreciated that references herein to the first effluentstream comprising at least 5 wt % more aromatic rings than the feed isintended to mean that the total number of aromatic rings in the firsteffluent stream should exceed the total number of aromatic rings in thefeed by at least 5 wt %. For example, if the feed contains 1 wt % ofaromatic rings, the first effluent stream will contain at least 6 wt %of aromatic rings. Changes in substituents on any aromatic rings betweenthe feed and the first effluent stream are not included in thiscalculation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of a process for converting methane to higherhydrocarbons according to a first example of the invention.

FIG. 2 is a flow diagram of a process for converting methane to higherhydrocarbons according to a second example of the invention.

FIG. 3 is a flow diagram of a process for converting methane to higherhydrocarbons according to a fourth example of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

This application describes a process for converting methane to liquidhydrocarbons by subjecting a feed containing methane, typically togetherwith at least one of H₂, H₂O, CO and CO₂, to a dehydrocyclization stepunder conditions effective to convert methane to aromatic hydrocarbonsand produce a first effluent stream comprising aromatic hydrocarbons andhydrogen, wherein the first effluent stream comprises at least 5 wt %more aromatic hydrocarbons than the feed.

The first effluent stream is then subjected to a hydrogenation step inwhich at least part of the benzene or naphthalene from said firsteffluent stream is reacted with hydrogen, preferably at least part ofthe hydrogen from said first effluent stream, to produce a secondeffluent stream having a reduced benzene or naphthalene content comparedwith the first effluent stream. At least one liquid hydrocarbonfraction, such as cyclohexane, cyclohexene,dihydronaphthalene(benzylcyclohexene), tetrahydronaphthalene(tetralin),hexahydronaphthalene(dicyclohexene), octahydronaphthalene, ordecahydronaphthalene(decalin), is recovered from the second effluentstream. If desired, the first effluent stream can be subjected to anaromatics alkylation step prior to or after hydrogenation of one or morearomatic hydrocarbon fractions.

Feedstock

Any methane-containing feedstock can be used in the process of theinvention but in general the present process is intended for use with anatural gas feedstock. Other suitable methane-containing feedstocksinclude those obtained from sources such as coal beds, landfills,agricultural or municipal waste fermentation, and/or refinery gasstreams.

Methane-containing feedstocks, such as natural gas, typically containcarbon dioxide and ethane in addition to methane. Ethane and otheraliphatic hydrocarbons that may be present in the feed can of course beconverted to desired aromatics products in the dehydrocyclization step.In addition, as will be discussed below, carbon dioxide can also beconverted to useful aromatics products either directly in thedehydrocyclization step or indirectly through conversion to methaneand/or other hydrocarbons in the hydrogen rejection step.

Nitrogen and/or sulfur impurities are also typically present inmethane-containing streams and are preferably removed, or reduced to lowlevels, prior to use of the streams in the process of the invention. Ingeneral, the feed to the dehydrocyclization step should contain lessthan 100 ppm, for example less than 10 ppm, such as less than 1 ppm eachof nitrogen and sulfur compounds.

In addition to methane, the feed to the dehydrocyclization steppreferably contains at least one of hydrogen, water, carbon monoxide andcarbon dioxide in order to assist in coke mitigation. These additivescan be introduced as separate co-feeds or can be present in the methanestream, such as, for example, where the methane stream is derived fromnatural gas containing carbon dioxide. Other suitable sources of carbondioxide include flue gases, LNG plants, hydrogen plants, ammonia plants,glycol plants and phthalic anhydride plants.

In one embodiment, the feed to the dehydrocyclization step containscarbon dioxide and comprises 90 to 99.9 mol %, such as 97 to 99 mol %methane and 0.1 to 10 mol %, such as 1 to 3 mol %, CO₂. In anotherembodiment, the feed to the dehydrocyclization step contains carbonmonoxide and comprises 80 to 99.9 mol %, such as 94 to 99 mol %, methaneand 0.1 to 20 mol %, such as 1 to 6 mol %, CO. In a third embodiment,the feed to the dehydrocyclization step contains steam and comprises 90to 99.9 mol %, such as 97 to 99 mol %, methane and 0.1 to 10 mol %, suchas 1 to 3 mol %, steam. In a fourth embodiment, the feed to thedehydrocyclization step contains hydrogen and comprises 80 to 99.9 mol %hydrocarbon, such as 95 to 99 mol %, methane and 0.1 to 20 mol %, suchas 1 to 5 mol %, hydrogen.

The feed to the dehydrocyclization step can also contain higherhydrocarbons than methane, including aromatic hydrocarbons. Such higherhydrocarbons can be added as separate co-feeds or can be present in themethane stream, such as, for example, when ethane is present in anatural gas feed. In general, however, the feed to thedehydrocyclization step contains less than 5 wt %, such as less than 3wt %, of C₃+ hydrocarbons.

Dehydrocyclization

In the dehydrocyclization step of the present process, the methanecontaining feed is contacted with a dehydrocyclization catalyst underconditions, normally non-oxidizing conditions and preferably reducingconditions, effective to convert the methane to higher hydrocarbons,including benzene and naphthalene. The principal reactions involved areas follows:2CH₄⇄C₂H₄+2H₂   (Reaction 1)6CH₄⇄C₆H₆+9H₂   (Reaction 2)10CH₄⇄C₁₀H₈+16H₂   (Reaction 3)

Carbon monoxide and/or dioxide that may be present in the feed improvescatalyst activity and stability by facilitating reactions such as:CO₂+coke→2CO   (Reaction 4)but negatively impacts equilibrium by allowing competing net reactions,such as;CO₂+CH₄⇄CO+2H₂   (Reaction 5).

Any dehydrocyclization catalyst effective to convert methane toaromatics can be used in the process of the invention, althoughgenerally the catalyst will include a metal component, particularly atransition metal or compound thereof, on an inorganic support.Conveniently, the metal component is present in an amount between 0.1%and 20%, such as between 1% and 10%, by weight of the total catalyst.

Suitable metal components for the catalyst include calcium, magnesium,barium, yttrrium, lanthanum, scandium, cerium, titanium, zirconium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,manganese, rhenium, iron, ruthenium, cobalt, rhodium, iridium, nickel,palladium, copper, silver, gold, zinc, aluminum, gallium, silicon,germanium, indium, tin, lead, bismuth and transuranium metals. Suchmetal components may be present in elemental form or as metal compounds,such as oxides, carbides, nitrides and/or phosphides, and may beemployed alone or in combination. Platinum and Osmium can also be usedas one of the metal component but, in general, are not preferred.

The inorganic support may be either amorphous or crystalline and anoxide, carbide or nitride of boron, aluminum, silicon, phosphorous,titanium, scandium, chromium, vanadium, magnesium, manganese, iron,zinc, gallium, germanium, yttrium, zirconium, niobium, molybdenum,indium, tin, barium, lanthanum, hafnium, cerium, tantalum, tungsten, orother transuranium elements. In addition, the support may be a porousmaterial, such as a microporous or mesoporous crystalline material.Suitable microporous crystalline materials include silicates,aluminosilicates, titanosilicates, aluminophosphates, metallophosphates,silicoaluminophosphates or their mixtures. Such microporous crystallinematerials include materials having the framework types MFI (e.g., ZSM-5and silicalite), MEL (e.g., ZSM-11), MTW (e.g., ZSM-12), TON (e.g.,ZSM-22), MTT (e.g., ZSM-23), FER (e.g., ZSM-35), MFS (e.g., ZSM-57), MWW(e.g., MCM-22, PSH-3, SSZ-25, ERB-1, ITQ-1, ITQ-2, MCM-36, MCM-49 andMCM-56), IWR (e.g., ITQ-24), KFI (e.g., ZK-5), BEA (e.g., zeolite beta),ITH (e.g., ITQ-13), MOR (e.g., mordenite), FAU (e.g., zeolites X, Y,ultrastabilized Y and dealuminized Y), LTL (e.g., zeolite L), IWW (e.g.,ITQ-22), VFI (e.g., VPI-5), AEL (e.g., SAPO-11), AFI (e.g., ALPO-5) andAFO (SAPO-41), as well as materials such as MCM-68, EMM-1, EMM-2,ITQ-23, ITQ-24, ITQ-25, ITQ-26, ETS-2, ETS-10, SAPO-17, SAPO-34 andSAPO-35. Suitable mesoporous materials include MCM-41, MCM-48, MCM-50and SBA-15.

Examples of preferred catalysts include molybdenum, tungsten, rheniumand compounds and combinations thereof on ZSM-5, silica or alumina.

The metal component can be dispersed on the inorganic support by anymeans well known in the art such as co-precipitation, incipient wetness,evaporation, impregnation, spray-drying, sol-gel, ion-exchange, chemicalvapor deposition, diffusion and physical mixing. In addition, theinorganic support can be modified by known methods, such as, forexample, steaming, acid washing, caustic washing and/or treatment withsilicon-containing compounds, phosphorus-containing compounds, and/orelements or compounds of Groups 1, 2, 3 and 13 of the Periodic Table ofElements. Such modifications can be used to alter the surface activityof the support and hinder or enhance access to any internal porestructure of the support.

The dehydrocyclization step can be conducted over a wide range ofconditions including a temperature of 400° C. to 1200° C., such as 500°C. to 975° C., for example 600° C. to 950° C., a pressure of 1 kPa to1000 kPa, such as 10 to 500 kPa, for example 50 kPa to 200 kPa and aweight hourly space velocity of 0.01 to 1000 hr⁻¹, such as 0.1 to 500hr⁻¹, for example 1 to 20 hr⁻¹. Conveniently, the dehydrocyclizationstep is conducted in the absence of O₂.

The dehydrocyclization step can be conducted in one or more fixed beds,moving beds or fluidized bed reactors, with catalyst regeneration beingconducted in-situ or ex-situ with air, oxygen, carbon dioxide, carbonmonoxide, water, hydrogen or combinations thereof.

The dehydrocyclization reaction is endothermic and, hence where thereaction is conducted in a plurality of stages, it may be necessary toemploy interstage heating to return the feed to the required reactiontemperature. The fuel required to provide the interstage heating isconveniently obtained by removing and combusting a sidestream from thedehydrocyclization effluent, after separation of the aromaticcomponents. In addition, where the reaction occurs in the presence of amoving bed of catalyst, a portion or all of the heat may be supplied bywithdrawing a portion of the catalyst from the bed, heating the catalystby, for example, combustion of coke on the catalyst, and then returningthe heated catalyst to the moving catalyst bed.

The major components of the effluent from the dehydrocyclization stepare hydrogen, benzene, naphthalene, carbon monoxide, ethylene, andunreacted methane. Typically, the effluent contains at least 5 wt %,such as at least 10 wt %, for example at least 20 wt % more aromatichydrocarbons than the feed.

Aromatic Product Recovery/Treatment

The benzene and naphthalene can be separated from the dehydrocyclizationeffluent, typically by solvent extraction followed by fractionation, andthen sold directly as commodity chemicals. In addition, before or afterseparation from the dehydrocyclization effluent, some or all of thebenzene and/or naphthalene can be alkylated to produce, for example,toluene, xylenes and alkyl naphthalenes. In accordance with theinvention, however, at least part of the aromatic components of thedehydrocyclization effluent is subjected to hydrogenation to generateuseful liquid products such as cyclohexane, cyclohexene,dihydronaphthalene(benzylcyclohexene), tetrahydronaphthalene(tetralin),hexahydronaphthalene(dicyclohexene), octahydronaphthalene and/ordecahydronaphthalene(decalin). These products can be employed as fuelsand chemical intermediates and, in the case of tetralin and decalin, canbe used as the solvent for extracting the aromatic components from thedehydrocyclization effluent.

Where the dehydrocyclization process is conducted at a remote location,hydrogenation of the aromatic components can also be used as aneffective method for converting some of the hydrogen by-product intoliquid from, namely the liquid products of the hydrogenation process, tofacilitate transportation of the hydrogen to a location where it willhave more value. In this case, after transportation of the liquidproduct it may be economically viable to reconvert the liquid productback to the original aromatic components and hydrogen.

Aromatics Hydrogenation

Conveniently, at least part of the benzene and/or naphthalene is reactedwith hydrogen from the first effluent stream to produce one or more ofcyclohexane, cyclohexene, dihydronaphthalene(benzylcyclohexene),tetrahydronaphthalene(tetralin), hexahydronaphthalene(dicyclohexene),octahydronaphthalene, and decahydronaphthalene(decalin). Exemplaryreactions are the production of cyclohexene as follows:C₆H₆+2H₂→C₆H₁₀;the production of dihydronaphthalene as follows:C₁₀H₈+H₂→C₁₀H₁₀;the production of tetrahydronaphthalene as follows:C₁₀H₈+2H₂→C₁₀H₁₂;the production of hexahydronaphthalene as follows: andC₁₀H₈+3H₂→C₁₀H₁₄;the production of decahydronaphthalene as follows:C₁₀H₈+5H₂→C₁₀H₁₈.

A portion of the naphthalene may also be subjected to hydrogenation ofone ring followed by hydrocracking of the ring to produce variousalkylated one ring aromatic species such as ethylbenzene, xylenes,cumene, tri-methylbenzene, butylbenzene, diethylbenzene,methylethylbenzene and other typical isomers. Although typically of lessvalue, the benzene and/or naphthalene can also be hydrocracked to adistribution of carbon numbers and isomers of normal and isoparaffins.

Conveniently, at least part of the benzene and/or naphthalene can befurther reacted with produced cyclohexene to respectively formcyclohexyl benzene or cyclohexyl naphthalene.

There are several general reasons why it is advantageous to performhydrogenation of the benzene and naphthalene:

-   -   Consumption of a portion of the coproduced hydrogen reduces the        need for other methods of utilizing the hydrogen—remote, low        value hydrogen is converted to chemically bound, transportable        hydrogen    -   Fuels with higher hydrogen content are preferred as lower CO2        emissions per energy content are possible    -   Very low sulfur fuels are produced which are desirable for both        conventional usages as well as fuel cell applications where        sulfur is particularly detrimental    -   Benzene is converted to materials with:        -   Reduced health and environment issues        -   Cyclohexane has high value as feedstock for polymer            production        -   Cyclohexene can be used directly as a monomer for polymer            products or can be easily reacted to produce materials such            as cyclohexanone or cyclohexylbenzene    -   Naphthalene is converted to materials with:        -   Reduced health and environment issues        -   Potential to solidify at ambient temperature is eliminated        -   Cyclohexane has high value as feedstock for polymer            production        -   Dihydronaphthalene(benzylcyclohexene),            hexahydronaphthalene(dicyclohexene), octahydronaphthalene            can be used directly as a monomer for polymer products or            can be easily reacted to produce materials such as            benzylcyclohexanone or dicyclohexanone        -   Decalin can have high value as a jet fuel, particularly as a            high density fuel for military aircraft        -   hydrocracked to a distribution of carbon numbers and isomers            of normal and isoparaffins can yield low aromatic, low            sulfur diesel and jet fuel        -   hydrogenation of one ring followed by hydrocracking of the            ring to produce various alkylated one ring aromatic species,            such as ethylbenzene, xylenes, cumene, tri-methylbenzene,            butylbenzene, diethylbenzene, methylethylbenzene and other            typical isomers, yields a product stream that can be used            for chemicals recovery or production of high octane, low            sulfur internal combustion engine fuel

The hydrogenation is conveniently, but not necessarily, conducted afterseparation of the aromatic components from the dehydrocyclizationeffluent and conveniently employs part of the hydrogen generated by thedehydrocyclization reaction. If the feed contains nitrogen and/or sulfurimpurities, it may be desirable to treat the hydrogen generated by thedehydrocyclization reaction to reduce the levels of sulfur and nitrogencompounds in the hydrogen before the hydrogen is used to hydrogenate thearomatic components from the dehydrocyclization effluent. It may also bedesirable to remove at least a portion of the unreacted methane andother light hydrocarbons from the hydrogen stream prior to use. Hydrogencontent of the stream may also be improved by steam reforming of thestream prior to use in hydrogenation. The hydrogen stream after use forhydrogenation may be enriched in C₁-C₅ paraffins which are suitable forrecycle to the dehydrocyclization reactor.

Suitable aromatic hydrogenation processes are well known in the art andtypically employ a catalyst comprising Ni, Pd, Pt, Ni/Mo or sulfidedNi/Mo supported on alumina or silica support or other high surface areainorganic support. Suitable operating conditions for the hydrogenationprocess include a temperature of 300 to 1,000° F. (150 to 540° C.), suchas 500 to 700° F. (260 to 370° C.), a pressure of t 50 to 2,000 psig(445 to 13890 kPa), such as 100 to 500 psig (790 to 3550 kPa) and a WHSVof 0.5 to 50 hr⁻¹, such as 2 to 10 hr⁻¹.

Partial hydrogenation to leave one or more olefinic carbon-carbon bondsin the product may also be desirable so as to produce materials suitablefor polymerization or other downstream chemical conversion. Suitablepartial hydrogenation processes are well known in the art and typicallyemploy a catalyst comprising noble metals with ruthenium being preferredsupported on metallic oxides, such as La₂O₃—ZnO. Homogeneous noble metalcatalyst systems can also be used. Examples of partial hydrogenationprocesses are disclosed in U.S. Pat. Nos. 4,678,861; 4,734,536;5,457,251; 5,656,761; 5,969,202; and 5,973,218, the entire contents ofwhich are incorporated herein by reference.

An alternative hydrogenation process involves hydrocracking of thenaphthalene component to produce alkylbenzenes over a catalyst such assulfided Ni/W or sulfided Ni supported on an amorphous aluminosilicateor a zeolite, such as zeolite X, zeolite Y or zeolite beta or other highsurface area inorganic support. Suitable operating conditions for lowpressure hydrocracking include a temperature of 300 to 1,000° F. (150 to540° C.), such as 500 to 700° F. (260 to 370° C.), a pressure of 50 to2,000 psig (445 to 13890 kPa), such as 100 to 500 psig (790 to 3550 kPa)and a WHSV of 0.5 to 50 hr⁻¹, such as 2 to 10 hr⁻¹.

An alternative hydrogenation process involves hydrocracking of thebenzene or naphthalene component to produce normal or branched paraffinsover a catalyst such as Group VIII metals, with Pt and Ir beingpreferred, supported on an amorphous aluminosilicate or a zeolite, suchas zeolite X, zeolite Y or zeolite beta or other high surface areainorganic support. Suitable operating conditions for low pressurehydrocracking include a temperature of 300 to 1,000° F. (150 to 540°C.), such as 500 to 700° F. (260 to 370° C.), a pressure of 50 to 2,000psig (445 to 13890 kPa), such as 100 to 500 psig (790 to 3550 kPa) and aWHSV of 0.5 to 50 hr⁻¹, such as 2 to 10 hr⁻¹.

Aromatics Alkylation

Alkylation of aromatic compounds such as benzene and naphthalene is wellknown in the art and typically involves reaction of an olefin, alcoholor alkyl halide with the aromatic species in the gas or liquid phase inthe presence of an acid catalyst. Suitable acid catalysts include mediumpore zeolites (i.e., those having a Constraint Index of 2-12 as definedin U.S. Pat. No. 4,016,218), including materials having the frameworktypes MFI (e.g., ZSM-5 and silicalite), MEL (e.g., ZSM-11), MTW (e.g.,ZSM-12), TON (e.g., ZSM-22), MTT (e.g., ZSM-23), MFS (e.g., ZSM-57) andFER (e.g., ZSM-35) and ZSM-48, as well as large pore zeolites (i.e.,those having a Constraint Index of less than 2) such as materials havingthe framework types BEA (e.g., zeolite beta), FAU (e.g., ZSM-3, ZSM-20,zeolites X, Y, ultrastabilized Y and dealuminized Y), MOR (e.g.,mordenite), MAZ (e.g., ZSM-4), MEI (e.g., ZSM-18) and MWW (e.g., MCM-22,PSH-3, SSZ-25, ERB-1, ITQ-1, ITQ-2, MCM-36, MCM-49 and MCM-56).

In one embodiment of the present process, benzene is recovered from thedehydrocyclization effluent and then alkylated with an olefin, such asethylene produced as a by-product of a hydrogen rejection step employingethanation/methanation. Typical conditions for carrying out the vaporphase alkylation of benzene with ethylene include a temperature of from650 to 900° F. (343 to 482° C.), a pressure of atmospheric to 3000 psig(100 to 20,800 kPa), a WHSV based on ethylene of from 0.5 to 2.0 hr⁻¹and a mole ratio of benzene to ethylene of from 1:1 to 30:1. Liquidphase alkylation of benzene with ethylene may be carried out at atemperature between 300 and 650° F. (150 to 340° C.), a pressure up toabout 3000 psig (20,800 kPa), a WHSV based on ethylene of from 0.1 to 20hr⁻¹ and a mole ratio of benzene to ethylene of from 1:1 to 30:1.

Preferably, the benzene ethylation is conducted under at least partialliquid phase conditions using a catalyst comprising at least one ofzeolite beta, zeolite Y, MCM-22, PSH-3, SSZ-25, ERB-1, ITQ-1, ITQ-2,ITQ-13, ZSM-5 MCM-36, MCM-49 and MCM-56.

The benzene ethylation can be conducted at the site of thedehydrocyclization/hydrogen rejection process or the benzene can beshipped to another location for conversion to ethylbenzene. Theresultant ethylbenzene can then be sold, used as a precursor in, forexample, the production of styrene or isomerized by methods well knownin the art to mixed xylenes.

In another embodiment of the present process, the alkylating agent ismethanol or dimethylether (DME) and is used to alkylate benzene and/ornaphthalene recovered from the dehydrocyclization effluent to producetoluene, xylenes, methylnaphthalenes and/or dimethylnaphthalenes. Wherethe methanol or DME is used to alkylate benzene, this is convenientlyeffected in presence of catalyst comprising a medium pore zeolite, suchas ZSM-5, Beta, ITQ-13, MCM-22, MCM-49, ZSM-11, ZSM-12, ZSM-22, ZSM-23,ZSM-35, and ZSM-48, which has been modified by steaming so as to have aDiffusion Parameter for 2,2 dimethylbutane of 0.1-15 sec⁻¹ when measuredat a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr(8 kPa). Such a process is selective to the production of para-xyleneand is described in, for example, U.S. Pat. No. 6,504,272, incorporatedherein by reference. Where the methanol is used to alkylate naphthalene,this is conveniently effected in the presence of a catalyst comprisingZSM-5, MCM-22, PSH-3, SSZ-25, ERB-1, ITQ-1, ITQ-2, ITQ-13, MCM-36,MCM-49 or MCM-56. Such a process can be used to selectively produce2,6-dimethylnaphthalene and is described in, for example, U.S. Pat. Nos.4,795,847 and 5,001,295, incorporated herein by reference.

Where methanol or DME is used as an alkylating agent in the process ofthe invention, it can be provided as a separate feed to the process orcan at least partly be generated in situ by adding a carbondioxide-containing feed gas, such as a natural gas stream, to part orall of the effluent from the dehydrocyclization step. In particular, thedehydrocyclization effluent, prior to or after separation of thearomatic components, can be fed to a reverse shift reactor and reactedwith the carbon dioxide-containing feed under conditions to increase thecarbon monoxide content of the effluent by reactions, such as Reaction 5and the following reverse water gas shift reaction:CO₂+H₂⇄CO+H₂O   (Reaction 6)

In addition, methane and CO₂ and/or steam may be fed to a reverse shiftreactor to generate syngas which can then be mixed with a portion of thedehydrocyclization effluent to adjust the H₂/CO/CO₂ ratios as requiredfor the alkylation step.

Typically, the reverse shift reactor contains a catalyst comprising atransition metal on a support, such as Fe, Ni, Cr, Zn and Cu on alumina,silica or titania, and is operated under conditions including atemperature of 500° C. to 1200° C., such as 600° C. to 1000° C., forexample 700° C. to 950° C. and a pressure of 1 kPa to 10,000 kPa, suchas 2,000 kPa to 10,000 kPa, for example 3000 kPa to 5,000 kPa. Gashourly space velocities may vary depending upon the type of processused, but generally the gas hourly space velocity of flow of gas throughthe catalyst bed is in the range of 50 hr⁻¹ to 50,000 hr⁻¹, such as 250hr⁻¹ to 25,000 hr⁻¹, more preferably 500 hr⁻¹ to 10,000 hr⁻¹.

The effluent from the reverse shift reactor can then be fed to analkylation reactor operating under conditions to cause reactions such asthe following to occur:CO+2H₂⇄CH₃OH   (Reaction 7)CH₃OH+C₆H₆→toluene+2H₂O   (Reaction 8)2CH₃OH+C₆H₆→xylenes+2H₂O   (Reaction 9)

Suitable conditions for such an alkylation reactor would include atemperature of 100 to 700° C., a pressure of 1 to 300 atmospheres (100to 30,000 kPa), and a WHSV for the aromatic hydrocarbon of 0.01 to 100hr⁻¹. A suitable catalyst would comprise a molecular sieve having aconstraint index of 1 to 12, such as ZSM-5, typically together with oneor metals or metal oxides, such as copper, chromium and/or zinc oxide.

Preferably, where the alkylation catalyst includes a molecular sieve,the latter is modified to change its diffusion characteristics such thatthe predominant xylene isomer produced by Reaction 9 is paraxylene.Suitable means of diffusion modification include ex-situ or in-situdeposition of coke, silicon, and/or metal oxides on the surface or inthe pore mouths of the molecular sieve. Also preferred is that an activemetal be incorporated into the molecular sieve so as to saturate morehighly reactive species, such as olefins, which may be generated asby-products and which could otherwise cause catalyst deactivation.

The effluent from the alkylation reactor could then be fed to aseparation section in which the aromatic products would initially beseparated from the hydrogen and other low molecular weight materials,conveniently by solvent extraction. The aromatics products could then befractionated into a benzene fraction, a toluene fraction, a C₈ fractionand a heavy fraction containing naphthalene and alkylated naphthalenes.The C₈ aromatic fraction could then be fed to a crystallization orsorption process to separate the valuable p-xylene component and theremaining mixed xylenes either sold as product or fed to anisomerization loop to generate more p-xylene. The toluene fraction couldeither be removed as saleable product, recycled to the alkylationreactor or fed to a toluene disproportionation unit and preferably aselective toluene disproportionation unit for the preparation ofadditional p-xylene.

Hydrogen Rejection

Hydrogen is a major component of the dehydrocyclization effluent and,although part of this hydrogen is used in the hydrogenation of thearomatic products and possibly in the optional alkylation step, theeffluent is subjected to a hydrogen rejection step to reduce thehydrogen content of the effluent before the unreacted methane isrecycled to the dehydrocyclization step and to maximize feedutilization. Typically the hydrogen rejection step comprises reacting atleast part of the hydrogen in the dehydrocyclization effluent with anoxygen-containing species to produce water and a second effluent streamhaving a reduced hydrogen content compared with the first(dehydrocyclization) effluent stream.

Conveniently, the hydrogen rejection step includes (i) methanationand/or ethanation, (ii) a Fischer-Tropsch process, (iii) synthesis of C₁to C₃ alcohols, particularly methanol, and other oxygenates, (iv)synthesis of light olefins by way of a methanol or dimethyl etherintermediate and/or (v) selective hydrogen combustion. These steps maybe employed sequentially to gain the greatest benefit; for exampleFischer-Tropsch may first be employed to yield a C₂+ enriched streamfollowed by methanation to achieve high conversion of the H₂.

Methanation/Ethanation

In one embodiment the hydrogen rejection step comprises reaction of atleast part of the hydrogen in the dehydrocyclization effluent withcarbon dioxide to produce methane and/or ethane according to thefollowing net reactions:CO₂+4H₂⇄CH₄+2H₂O   (Reaction 10)2CO₂+7H₂⇄C₂H₆+4H₂O   (Reaction 11)

The carbon dioxide employed is conveniently part of a natural gas streamand preferably the same natural gas stream used as the feed to thedehydrocyclization step. Where the carbon dioxide is part of amethane-containing stream, the CO₂:CH₄ of the stream is convenientlymaintained between 1:1 and 0.1:1. Mixing of the carbondioxide-containing stream and the dehydrocyclization effluent isconveniently achieved by supplying the gaseous feeds to the inlet of ajet ejector.

The hydrogen rejection step to produce methane or ethane normallyemploys a H₂:CO₂ molar ratio close to the stoichiometric proportionsrequired for the desired Reaction 10 or Reaction 11, although smallvariations can be made in the stoichiometric ratio if it is desired toproduce a CO₂-containing or H₂-containing second effluent stream. Thehydrogen rejection step to produce methane or ethane is convenientlyeffected in the presence of a bifunctional catalyst comprising a metalcomponent, particularly a transition metal or compound thereof, on aninorganic support. Suitable metal components comprise copper, iron,vanadium, chromium, zinc, gallium, nickel, cobalt, molybdenum,ruthenium, rhodium, palladium, silver, rheniuni, tungsten, iridium,platinum, gold, gallium and combinations and compounds thereof. Theinorganic support may be an amorphous material, such as silica, aluminaor silica-alumina, or like those listed for the dehydroaromatizationcatalyst. In addition, the inorganic support may be a crystallinematerial, such as a microporous or mesoporous crystalline material.Suitable porous crystalline materials include the aluminosilicates,aluminophosphates and silicoaluminophosphates listed above for thedehydrocyclization catalyst.

The hydrogen rejection step to produce methane and/or ethane can beconducted over a wide range of conditions including a temperature of100° C. to 900° C., such as 150° C. to 500° C., for example 200° C. to400° C., a pressure of 200 kPa to 20,000 kPa, such as 500 to 5000 kPaand a weight hourly space velocity of 0.1 to 10,000 hr⁻¹, such as 1 to1,000 hr⁻¹. CO₂ conversion levels are typically between 20 and 100% andpreferably greater than 90%, such as greater than 99%. This exothermicreaction may be carried out in multiple catalyst beds with heat removalbetween beds. In addition, the lead bed(s) may be operated at highertemperatures to maximize kinetic rates and the tail beds(s) may beoperated at lower temperatures to maximize thermodynamic conversion.

The main products of the reaction are water and, depending on the H₂:CO₂molar ratio, methane, ethane and higher alkanes, together with someunsaturated C₂ and higher hydrocarbons. In addition, some partialhydrogenation of the carbon dioxide to carbon monoxide is preferred.After removal of the water, the methane, carbon monoxide, any unreactedcarbon dioxide and higher hydrocarbons can be fed directly to thedehydrocyclization step to generate additional aromatic products.

Fischer-Tropsch Process

In another embodiment the hydrogen rejection step comprises reaction ofat least part of the hydrogen in the dehydrocyclization effluent withcarbon monoxide according to the Fischer-Tropsch process to produce C₂to C₅ paraffins and olefins.

The Fischer-Tropsch process is well known in the art, see for example,U.S. Pat. Nos. 5,348,982 and 5,545,674 incorporated herein by reference.The process typically involves the reaction of hydrogen and carbonmonoxide in a molar ratio of 0.5:1 to 4:1, preferably 1.5:1 to 2.5:1, ata temperature of 175° C. to 400° C., preferably 180° C. to 240° C. and apressure of 1 to 100 bar (100 to 10,000 kPa), preferably 10 to 40 bar(1,000 to 4,000 kPa), in the presence of a Fischer-Tropsch catalyst,generally a supported or unsupported Group VIII, non-noble metal, e.g.,Fe, Ni, Ru, Co, with or without a promoter, e.g. ruthenium, rhenium,hafnium, zirconium, titanium. Supports, when used, can be refractorymetal oxides such as Group IVB, i.e., titania, zirconia, or silica,alumina, or silica-alumina. In one embodiment, the catalyst comprises anon-shifling catalyst, e.g., cobalt or ruthenium, preferably cobalt,with rhenium or zirconium as a promoter, preferably cobalt and rheniumsupported on silica or titania, preferably titania.

In another embodiment, the hydrocarbon synthesis catalyst comprises ametal, such as Cu, Cu/Zn or Cr/Zn, on the ZSM-5 and the process isoperated to generate significant quantities of single-ring aromatichydrocarbons. An example of such a process is described in Study ofPhysical Mixtures of Cr ₂0₃ —ZnO and ZSM-5 Catalysts for theTransformation of Syngas into Liquid Hydrocarbons by Jose Erena; Ind.Eng. Chem Res. 1998, 37, 1211-1219, incorporated herein by reference.

The Fischer-Tropsch liquids, i.e., C₅+, are recovered and light gases,e.g., unreacted hydrogen and CO, C₁ to C₃ or C₄ and water are separatedfrom the heavier hydrocarbons. The heavier hydrocarbons can then berecovered as products or fed to the dehydrocyclization step to generateadditional aromatic products.

The carbon monoxide required for the Fischer-Tropsch reaction can beprovided wholly or partly by the carbon monoxide present in or cofedwith the methane-containing feed and generated as a by-product in thedehydrocyclization step. If required, additional carbon monoxide can begenerated by feeding carbon dioxide contained, for example, in naturalgas, to the Fischer-Tropsch reaction whereby the carbon dioxide isconverted to carbon monoxide by the reverse water gas shift reaction.

Alcohol Synthesis

In a further embodiment the hydrogen rejection step comprises reactionof at least part of the hydrogen in the dehydrocyclization effluent withcarbon monoxide to produce C₁ to C₃ alcohols, and particularly methanol.The production of methanol and other oxygenates from synthesis gas isalso well-known and is described in, for example, in U.S. Pat. Nos.6,114,279; 6,054,497; 5,767,039; 5,045,520; 5,254,520; 5,610,202;4,666,945; 4,455,394; 4,565,803; 5,385,949, the descriptions of whichare incorporated herein by reference. Typically, the synthesis gasemployed has a molar ratio of hydrogen (H₂) to carbon oxides (CO+CO₂) inthe range of from 0.5:1 to 20:1, preferably in the range of from 2:1 to10:1, with carbon dioxide optionally being present in an amount of notgreater than 50% by weight, based on total weight of the syngas.

The catalyst used in the methanol synthesis process generally includesan oxide of at least one element selected from the group consisting ofcopper, silver, zinc, boron, magnesium, aluminum, vanadium, chromium,manganese, gallium, palladium, osmium and zirconium. Conveniently, thecatalyst is a copper based catalyst, such as in the form of copperoxide, optionally in the presence of an oxide of at least one elementselected from silver, zinc, boron, magnesium, aluminum, vanadium,chromium, manganese, gallium, palladium, osmium and zirconium.Conveniently, the catalyst contains copper oxide and an oxide of atleast one element selected from zinc, magnesium, aluminum, chromium, andzirconium. In one embodiment, the methanol synthesis catalyst isselected from the group consisting of: copper oxides, zinc oxides andaluminum oxides. More preferably, the catalyst contains oxides of copperand zinc.

The methanol synthesis process can be conducted over a wide range oftemperatures and pressures. Suitable temperatures are in the range offrom 150° C. to 450° C., such as from 175° C. to 350° C., for examplefrom 200° C. to 300° C. Suitable pressures are in the range of from1,500 kPa to 12,500 kPa, such as from 2,000 kPa to 10,000 kPa, forexample 2,500 kPa to 7,500 kPa. Gas hourly space velocities varydepending upon the type of process that is used, but generally the gashourly space velocity of flow of gas through the catalyst bed is in therange of from 50 hr⁻¹ to 50,000 hr⁻¹, such as from 250 hr⁻¹ to 25,000hr⁻¹, more preferably from 500 hr⁻¹ to 10,000 hr⁻¹. This exothermicreaction may be carried out in either fixed or fluidized beds, includingmultiple catalyst beds with heat removal between beds. In addition, thelead bed(s) may be operated at higher temperatures to maximize kineticrates and the tail beds(s) may be operated at lower temperatures tomaximize thermodynamic conversion.

The resultant methanol and/or other oxygenates can be sold as a separateproduct, can be used to alkylate the aromatics generated in thedehydrocyclization step to higher value products, such as xylenes, orcan be used as a feedstock for the production of lower olefins,particularly ethylene and propylene. The conversion of methanol toolefins is a well-known process and is, for example, described in U.S.Pat. No. 4,499,327, incorporated herein by reference.

Selective Hydrogen Combustion

In yet another embodiment, the hydrogen rejection step comprisesselective hydrogen combustion, which is a process in which hydrogen in amixed stream is reacted with oxygen to form water or steam withoutsubstantially reacting hydrocarbons in the stream with oxygen to formcarbon monoxide, carbon dioxide, and/or oxygenated hydrocarbons.Generally, selective hydrogen combustion is carried out in the presenceof an oxygen-containing solid material, such as a mixed metal oxide,that will release a portion of the bound oxygen to the hydrogen.

One suitable selective hydrogen combustion process is described in U.S.Pat. No. 5,430,210, incorporated herein by reference, and comprisescontacting at reactive conditions a first stream comprising hydrocarbonand hydrogen and a second stream comprising oxygen with separatesurfaces of a membrane impervious to non-oxygen containing gases,wherein said membrane comprises a metal oxide selective for hydrogencombustion, and recovering selective hydrogen combustion product. Themetal oxide is typically a mixed metal oxide of bismuth, indium,antimony, thallium and/or zinc.

U.S. Pat. No. 5,527,979, incorporated herein by reference, describes aprocess for the net catalytic oxidative dehydrogenation of alkanes toproduce alkenes. The process involves simultaneous equilibriumdehydrogenation of alkanes to alkenes and the selective combustion ofthe hydrogen formed to drive the equilibrium dehydrogenation reactionfurther to the product alkenes. In particular, the alkane feed isdehydrogenated over an equilibrium dehydrogenation catalyst in a firstreactor, and the effluent from the first reactor, along with oxygen, isthen passed into a second reactor containing a metal oxide catalystwhich serves to selectively catalyze the combustion of hydrogen. Theequilibrium dehydrogenation catalyst may comprise platinum and theselective metal oxide combustion catalyst may contain bismuth, antimony,indium, zinc, thallium, lead and tellurium or a mixture thereof.

U.S. Patent Application Publication No. 2004/0152586, published Aug. 5,2004 and incorporated herein by reference, describes a process forreducing the hydrogen content of the effluent from a cracking reactor.The process employs a catalyst system comprising (1) at least one solidacid cracking component and (2) at least one metal-based selectivehydrogen combustion component consisting essentially of (a) a metalcombination selected from the group consisting of:

-   i) at least one metal from Group 3 and at least one metal from    Groups 4-15 of the Periodic Table of the Elements;-   ii) at least one metal from Groups 5-15 of the Periodic Table of the    Elements, and at least one metal from at least one of Groups 1, 2,    and 4 of the Periodic Table of the Elements;-   iii) at least one metal from Groups 1-2, at least one metal from    Group 3, and at least one metal from Groups 4-15 of the Periodic    Table of the Elements; and-   iv) two or more metals from Groups 4-15 of the Periodic Table of the    Elements and (b) at least one of oxygen and sulfur, wherein the at    least one of oxygen and sulfur is chemically bound both within and    between the metals.

The selective hydrogen combustion reaction of the present invention isgenerally conducted at a temperature in the range of from 300° C. to850° C. and a pressure in the range of from 1 atm to 20 atm (100 to 2000kPa).

The invention will now be more particularly described with reference tothe following Examples and the accompanying drawings.

EXAMPLE 1

In a first example of the invention that is illustrated schematically inFIG. 1, a feed 11 comprising 100 kg of methane is fed to adehydrocyclization reactor 12 that contains a catalyst comprising 3 wt %Mo on HZSM-5 (silica to alumina molar ratio of 25) and is operated at atemperature of about 800° C., a WHSV of 1 and a pressure of 20 psia (138kPa). The effluent 13 from the dehydrocyclization reactor 12 is made upof an aromatic component comprising 10.78 kg of benzene and 2.45 kg ofnaphthalene and a fuel gas component comprising 79.62 kg of unreactedmethane, 4.01 kg of hydrogen and 0.85 kg of ethylene.

The effluent 13 is fed to a first separator 14, where the effluent isseparated, conveniently by solvent extraction, into the aromaticcomponent 15 and the fuel gas component 16. The fuel gas component isthen fed to a second separator 17, where a hydrogen stream 18 comprising1.05 kg of hydrogen is removed from the component 16 to generate a fuelgas product 19 comprising 79.62 kg of unreacted methane, 2.96 kg ofhydrogen and 0.85 kg of ethylene. The hydrogen stream 18 and thearomatic component 15 are then fed to a hydrogenation reactor 21 wherethe aromatic component is hydrogenated to produce a liquid hydrocarbonproduct 22 comprising 11.6 kg of cyclohexane and 2.7 kg of decalin.

The liquid hydrocarbon product 22 can be recovered for use as, forexample, a fuel or can be transported to a more desirable location, forexample a chemical plant, where the product can be fed to adehydrogenation reactor 23 for conversion and separation back to anaromatic product stream 24 comprising 10.78 kg of benzene and 2.45 kg ofnaphthalene and a hydrogen product stream 25 comprising 1.05 kg ofhydrogen.

EXAMPLE 2

In a second example of the invention illustrated schematically in FIG.2, the feed 31 comprises 114 kg of carbon dioxide in addition to 100 kgof methane and is initially fed to a methanation reactor 32 thatcontains a Cu/Zn catalyst and is operated at a temperature of 300° C., aWHSV of 1 and a pressure of 350 psia (2413 kPa). The effluent stream 33from the methanation reactor 32 is fed to a condenser 34, where 94 kgwater is recovered, and then the remaining effluent is fed to adehydrocyclization reactor 35 that contains a catalyst comprising 3 wt %Mo on HZSM-5 (silica to alumina molar ratio of 25) and is operated at atemperature of about 800° C., a WHSV of 1 and a pressure of 20 psia (138kPa). The effluent 36 from the dehydrocyclization reactor 35 is made upof an aromatic component comprising 78.8 kg of benzene and 17.2 kg ofnaphthalene and a fuel gas component comprising unreacted methane,hydrogen and ethylene.

The effluent 36 is fed to a first separator 37, where the effluent isseparated, conveniently by solvent extraction, into the aromaticcomponent 38 and the light gas component 39. The light gas component isthen fed to a second separator 41, where a hydrogen stream 42 is removedfrom light gas component and the remainder of the light gas is recycledas stream 43 to the methanation reactor 32. The hydrogen stream 42 andthe aromatic component 38 are then fed to a hydrogenation reactor 44where the aromatic component is hydrogenated to produce a liquidhydrocarbon product 45 comprising 84.9 kg of cyclohexane and 18.7 kg ofdecalin.

As in Example 1, the liquid hydrocarbon product 45 can be recovered foruse as, for example, a fuel or can be transported to a more desirablelocation, for example a chemical plant, where the product can be fed toa dehydrogenation reactor 46 for conversion and separation back to anaromatic product stream 47 comprising 78.8 kg of benzene and 17.2 kg ofnaphthalene and a hydrogen product stream 48.

EXAMPLE 3 (COMPARATIVE)

In this comparative example, the process of Example 2 is repeated butwithout the aromatic component 38 being hydrogenated and with all thelight gas component 39 being recycled to the methanation reactor 32. Inthis case, using a feed comprising 100 kg of methane and 114 kg ofcarbon dioxide, 149 kg water is recovered in the condenser 34 and thearomatic component 38 comprises 93 kg of benzene and 22 kg ofnaphthalene.

EXAMPLE 4

In a fourth example of the invention illustrated schematically in FIG.3, the feed 51 comprises 100 kg of methane and 70 kg of carbon dioxideand is initially to a methanation reactor 52 that contains a Cu/Zncatalyst and is operated at a temperature of 300° C., a WHSV of 1 and apressure of 350 psia (2413 kPa). The effluent stream 53 from themethanation reactor 52 is fed to a condenser 54, where 57 kg water isrecovered, and then the remaining effluent is fed to adehydrocyclization reactor 55 that contains a catalyst comprising 3 wt %Mo on HZSM-5 (silica to alumina molar ratio of 25) and is operated at atemperature of about 800° C., a WHSV of 1 and a pressure of 20 psia (138kPa). The effluent 56 from the dehydrocyclization reactor is made up ofan aromatic component comprising 69.8 kg of benzene and 15.9 kg ofnaphthalene and a fuel gas component comprising unreacted methane,hydrogen and ethylene.

The effluent 56 is fed to a first separator 57, where the effluent isseparated into a benzene product stream 58, which is recovered, anaphthalene stream 59 and the light gas component 61. The light gascomponent 61 is then fed to a second separator 62, where first andsecond hydrogen streams 63, 64 respectively are removed from light gascomponent and the remainder of the light gas is recycled as stream 65 tothe methanation reactor 52. The first hydrogen stream 63 and thenaphthalene stream 59 are then fed to a hydrogenation reactor 66 wherethe naphthalene stream is hydrogenated to produce a tetralin stream 67comprising 16.4 kg of tetralin. The tetralin stream 67 and the secondhydrogen stream 64 are then fed to a hydrocracker 68 where the tetralinis hydrocracked to produce a product stream 69 comprising 3.2 kgbenzene, 3.8 kg toluene, 4.4 kg xylenes, 5.5 kg propane and 11.2 kgethane.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations notnecessarily illustrated herein. For this reason, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

1. A process for converting methane to higher hydrocarbons, the processcomprising: (a) contacting a feed containing methane and at least one ofH₂, H₂O, CO and CO₂ with a dehydrocyclization catalyst under conditionseffective to convert said methane to aromatic hydrocarbons, includingbenzene and/or naphthalene, and produce a first effluent streamcomprising aromatic hydrocarbons and hydrogen, wherein said firsteffluent stream comprises at least 5 wt % more aromatic hydrocarbonsthan said feed; and (b) reacting at least part said aromatichydrocarbons from said first effluent stream with hydrogen to produce asecond effluent stream having a reduced benzene and/or naphthalenecontent compared with said first effluent stream, wherein said reactinghydrogenates and hydrocracks at least a portion of the naphthalene insaid first effluent stream to produce alkylated single ring aromaticcompounds.
 2. The process of claim 1, wherein at least part of thehydrogen used in (b) is from said first effluent stream.
 3. A processfor converting methane to higher hydrocarbons and hydrogen, the processcomprising: (a) contacting a feed containing methane with adehydrocyclization catalyst under non-oxidizing conditions effective toconvert said methane to aromatic hydrocarbons, including benzene and/ornaphthalene, and produce a first effluent stream comprising aromatichydrocarbons and hydrogen, wherein said first effluent stream comprisesat least 5 wt % more aromatic hydrocarbons than said feed; (b) reactingat least part of said aromatic hydrocarbons from said first effluentstream with at least part of the hydrogen from said first effluentstream, said reacting being conducted at a first location and producinga second effluent stream having a reduced benzene and/or naphthalenecontent compared with said first effluent stream; (c) transporting atleast part of said second effluent stream to a second location remotefrom said first location; and (d) dehydrogenating at least part of saidsecond effluent stream at said second location to produce hydrogen and athird effluent stream having an enhanced benzene and/or naphthalenecontent compared with said second effluent stream.
 4. The process ofclaim 3 further comprising reacting at least part of the hydrogen fromsaid first effluent stream with an oxygen containing species to producea further effluent stream comprising water and hydrocarbon; andrecycling at least part of the hydrocarbon in said further effluentstream to said contacting (a).
 5. The process of claim 4 wherein saidoxygen-containing species comprises carbon monoxide and/or carbondioxide.
 6. The process of claim 5 wherein said oxygen-containingspecies comprises carbon dioxide introduced to the process as part of anatural gas stream.
 7. The process of claim 6 wherein said natural gasstream also contains at least part of the methane in the feed in (a). 8.The process of claim 4 wherein said further effluent stream compriseswater and methane, ethane or a mixture of methane and ethane.
 9. Theprocess of claim 5 wherein oxygen-containing species comprises carbonmonoxide and said further effluent stream comprises one or more of C₂ toC₅ paraffins and olefins, single-ring aromatic hydrocarbons and C₁ to C₃alcohols.
 10. The process of claim 4 wherein said reacting at least partof the hydrogen from said first effluent stream with an oxygencontaining species includes selective hydrogen combustion.
 11. Theprocess of claim 3 wherein said feed in (a) also contains at least oneof H₂, H₂O, CO and CO₂.
 12. The process of claim 3 wherein said feed in(a) contains less than 5 wt % of C₃+ hydrocarbons.
 13. The process ofclaim 1 wherein said conditions in (a) are non-oxidizing conditions. 14.The process of claim 3 wherein said conditions in (a) include atemperature of about 400° C. to about 1200° C., a pressure of about 1kPa to about 1000 kPa and a weight hourly space velocity of about 0.1hr⁻¹ to about 1000 hr⁻¹.
 15. The process of claim 3 wherein saiddehydrocyclization catalyst in (a) comprises at least one of molybdenum,tungsten, rhenium, a molybdenum compound, a tungsten compound, and arhenium compound on ZSM-5, silica or an aluminum oxide.
 16. The processof claim 1 2 wherein said reacting (b) hydrocracks at least a portion ofthe naphthalene and/or benzene in said first effluent stream to producenormal and isoparaffins.
 17. The process of claim 3 and furthercomprising separating at least one aromatic hydrocarbon from said firsteffluent stream.
 18. The process of claim 17 wherein part of said firsteffluent stream remaining after separation of at least one aromatichydrocarbon is used as fuel to provide heat to said contacting (a). 19.The process of claim 3 and further comprising alkylating at least onearomatic hydrocarbon in said first effluent stream with an alkylatingagent.
 20. The process of claim 19 wherein said alkylating agentcomprises ethylene produced in said contacting (a).
 21. The process ofclaim 19 wherein said alkylating agent comprises carbon monoxide andhydrogen or a reaction product thereof.
 22. The process of claim 1further comprising reacting at least part of the hydrogen from saidfirst effluent stream with an oxygen containing species to produce afurther effluent stream comprising water and hydrocarbon; and recyclingat least part of the hydrocarbon in said further effluent stream to saidcontacting (a).
 23. The process of claim 22 wherein saidoxygen-containing species comprises carbon monoxide and/or carbondioxide.
 24. The process of claim 23 wherein said oxygen-containingspecies comprises carbon dioxide introduced to the process as part of anatural gas stream.
 25. The process of claim 24 wherein said natural gasstream also contains at least part of the methane in the feed in (a).26. The process of claim 22 wherein said further effluent streamcomprises water and methane, ethane or a mixture of methane and ethane.27. The process of claim 22 wherein oxygen-containing species comprisescarbon monoxide and said further effluent stream comprises one or moreof C₂ to C₅ paraffins and olefins, single-ring aromatic hydrocarbons andC₁ to C₃ alcohols.
 28. The process of claim 22 wherein said reacting atleast part of the hydrogen from said first effluent stream with anoxygen containing species includes selective hydrogen combustion. 29.The process of claim 22 wherein said feed in (a) also contains at leastone of H₂, H₂O, CO and CO₂.
 30. The process of claim 22 wherein saidfeed in (a) contains less than 5 wt % of C₃+ hydrocarbons.
 31. Theprocess of claim 22 wherein said conditions in (a) include a temperatureof about 400° C. to about 1200° C., a pressure of about 1 kPa to about1000 kPa and a weight hourly space velocity of about 0.1 hr⁻¹ to about1000 hr⁻¹.
 32. The process of claim 22 wherein said dehydrocyclizationcatalyst in (a) comprises at least one of molybdenum, tungsten, rhenium,a molybdenum compound, a tungsten compound, and a rhenium compound onZSM-5, silica or an aluminum oxide.
 33. The process of claim 22 andfurther comprising separating at least one aromatic hydrocarbon fromsaid first effluent stream.
 34. The process of claim 33 wherein part ofsaid first effluent stream remaining after separation of at least onearomatic hydrocarbon is used as fuel to provide heat to said contacting(a).
 35. The process of claim 22 and further comprising alkylating atleast one aromatic hydrocarbon in said first effluent stream with analkylating agent.
 36. The process of claim 35 wherein said alkylatingagent comprises ethylene produced in said contacting (a).
 37. Theprocess of claim 35 wherein said alkylating agent comprises carbonmonoxide and hydrogen or a reaction product thereof.
 38. A process forconverting methane to higher hydrocarbons and hydrogen, the processcomprising: (a) contacting a feed containing methane with adehydrocyclization catalyst under non-oxidizing conditions effective toconvert said methane to aromatic hydrocarbons, including benzene and/ornaphthalene, and produce a first effluent stream comprising aromatichydrocarbons and hydrogen, wherein said first effluent stream comprisesat least 5 wt % more aromatic hydrocarbons than said feed; (b) reactingat least part of said aromatic hydrocarbons from said first effluentstream with at least part of the hydrogen from said first effluentstream, said reacting being conducted at a first location and producinga second effluent stream having a reduced benzene and/or naphthalenecontent compared with said first effluent stream; (c) transporting atleast part of said second effluent stream to a second location remotefrom said first location; (d) dehydrogenating at least part of saidsecond effluent stream at said second location to produce hydrogen and athird effluent stream having an enhanced benzene and/or naphthalenecontent compared with said second effluent stream; and (e) reacting atleast part of the hydrogen from said first effluent stream with anoxygen containing species to produce a further effluent streamcomprising water and hydrocarbon; and recycling at least part of thehydrocarbon in said further effluent stream to said contacting (a).